DESCRIPTION, OVERALL (provided by applicant): Paradigm-shifting observations on aquaporin 2 (AQP2) and vasopressin receptor (V2R) trafficking, and vacuolar ATPase (V-ATPase) pH sensing and recycling have lead to new hypotheses to be addressed in this renewal. Project I will define VP-dependent and independent regulation of AQP2, and will identify proteins whose interaction with AQP2 is modified by phosphorylation to regulate trafficking. A novel role for AQP2 in regulating actin polymerization via interaction with RhoGAPS will be explored. Use of PDE5 inhibitors and statins to achieve VP-independent urine concentration will be tested as a potential strategy for future treatment of NDI. Project II will examine ligand induced conformational changes of the V2R at varying pH and tonicity using FRET techniques to dissect intra- and intermolecular protein interactions. Association of V2R with accessory proteins during internalization will be defined in cells expressing wild type and mutant V2R. Our novel observation that V2R interacts with the ESCRT protein Alix to accelerate V2R degradation will be pursued in these studies that address the regulation of body fluid homeostasis. Project III will pursue the breakthrough finding that the V-ATPase is an endosomal pH sensor by defining conformational changes in the V-ATPase tail that result in pH-dependent recruitment of small GTPases to membranes. It will identify pH sensitive residues on the luminal domains of the V-ATPase, and use albumin uptake to show relevance of the pH sensing mechanism to proximal tubule function. Project IV will elucidate downstream effectors (PKA, Epac) of soluble adenylate cyclase in modulating V-ATPase recycling and proton secretion in the epididymis, a model epithelium in which luminal acidic pH is critical for sperm maturation and storage. It will examine the role of cGMP-induced proton secretion in this tissue, and will address the exciting hypothesis that the V-ATPase is also an extracellular pH sensor that provides feedback control of luminal pH. These studies will allow a better understanding of male fertility, and uncover mechanisms that regulate V-ATPase in acidifying cells in general, including renal intercalated cells. The Microscopy Core B facility has been a major contributor to the success of this PPG. All projects gain considerable added value from extensive intellectual and technical collaborations that characterize our efforts to understand the relationship between cellular signaling, protein trafficking, and the responses of urogenital epithelial cells to their environment.